Glycosidases from tea-leaf (Camellia sinensis) and characterization of β-galactosidase

Abstract

A wide range of glycosidase activities could be detected in the acetone powder extract of tea leaves of Assam variety (Camellia sinensis). β-galactosidase (EC3.2.1.23) was found to be the most active among these enzymes at fermentation temperature needed for black tea processing and at acidic pH. Substrate staining showed only one detectable band for this enzyme. A 137-fold purification over acetone powder extract through gel filtration, DEAE-cellulose chromatography, centripep, and HPLC resulted in a preparation that was electrophoretically homogeneous. Substrate staining revealed the absence of any isozymic form of this enzyme. Determination of molecular mass by two independent methods (i.e., by gel filtration and by HPLC) showed the enzyme to be 60 kDa, SDS-PAGE data along with molecular mass data showed the active enzyme to be a monomer of 60 kDa. The enzyme was optimally active at 50°C and at pH 4.0 K_m for lactose was 4.2 mmol/L. Monovalent and divalent cations had no effect on the activity of the enzyme. Galactose, a hydrolytic product of this enzyme was found to be a competitive inhibitor with a K_i of 2.4 mmol/L. Galactose (5 mmol/L) could provide partial, but significant, protection against thermal inactivation. Several thiol modifying agents at low concentrations could rapidly inactivate the enzyme, inactivation could be reversed with dithiothreitol. Significant protection against inactivation by methyl methane thiosulphonate (MMTS) could be observed in presence of galactose indicating the leaves is indicative of their role in generating the characteristic flavor of black tea.